As a sensor for measuring a surface potential of a measured object, there is already known a variable capacitance potential sensor of mechanical type. FIG. 9 shows a principle of the variable capacitance potential sensor of mechanical type. A measured object 1099 has a potential V relative to a ground potential. A detection electrode 1021 is provided in an opposed relationship thereto, and a movable shutter 1025 is provided immediately above the detection electrode 1021. When the movable shutter 1025 moves, an electrostatic capacitance C between the measured object 1099 and the detection electrode 1025 shows a variation. In the detection electrode 1021, a charge Q is induced according to V and C. A current flowing between the detection electrode 1021 and the ground is detected by an ammeter 1060. As the charge Q induced in the detection electrode 1021 is given by Q=CV, a current flowing in the ammeter 1060 at a time t is given by i=dQ/dt=VdC/dt, and the potential V can be obtained if dC/dt is known. The dC/dt is a sensitivity of this sensor, and, as will be apparent from this relation, the sensitivity can be elevated by increasing the difference between the maximum and minimum values of C or reducing the time t of variation.
Such variable capacitance potential sensor of mechanical type, obtainable with the MEMS technology, is for example known in a following type (cf. U.S. Pat. No. 6,177,800). FIG. 10 illustrates a potential sensor 1001, which is constituted by a driver component 1010 and a sensor component 1020. These components can be prepared by the MEMS technology on a substrate 1004.
The driver component 1010 is formed by a suspension 1018 having a parallel hinge structure, and a comb-shaped electrostatic actuator 1012. The comb-shaped electrostatic actuator 1012 is a common mechanism for electrostatically driving a micro structure, and is constituted by a movable electrode 1013 supported by the suspension 1018 and a fixed electrode 1014 mounted on the substrate 1004. The comb-shaped electrostatic actuator 1012 is electrically connected to an electrostatic drive signal source 1050. The movable electrode 1013 is supported by the suspension 1018 so as to be movable in a lateral direction in the drawing. The comb-shaped electrodes of the movable electrode 1013 and those of the fixed electrode 1014 are mutually meshing and an electrostatic attractive force is exerted therebetween when a potential difference is given.
The driver component 1010 is connected to the sensor component 1020. A detection electrode assembly 1021 is fixed to the substrate 1004 and is capable of a capacitative coupling with a measured surface. The detection electrode assembly 1021 is constituted by a set of mutually separated individual detection electrodes (represented by 1021a, 1021b, 1021c). Individual detection probes are connected together, so that the individual signals are combined (superposed). The sensor component 1020 is further provided with a movable shutter 1025, which selectively covers the detection electrode assembly 1021. The movable shutter 1025 is mechanically connected to the driver component 1010, of which a linear displacement induces a corresponding displacement of the movable shutter 1025.
The movable shutter 1025 is provided with plural apertures 1024, which are so constructed as to selectively expose the detection electrode assembly 1021 through the apertures 1024 when the movable shutter 1025 is in a first position. The apertures 1024 are mutually separated by a distance corresponding to a distance between the detection electrodes. When the movable shutter 1025 is in a second position, the detection electrode assembly 1021 is covered by mask portions 1026 present between the apertures 1024. Stated differently, when the movable shutter 1025 is in the first position, the capacitative coupling by the detection electrode assembly 1021 is enabled. On the other hand, when the movable shutter 1025 is in the second position, the detection electrode assembly 1021 is masked and prevented from the capacitative coupling. A current generated in the detection electrode assembly is outputted to a lead electrode 1028 and is amplified by an amplifier 1060.
However, in the MEMS potential sensor of the aforementioned structure, the detection sensitivity cannot be made sufficiently high because an effective area of the detection electrode cannot be made large as will be explained in the following with reference to FIG. 11. The detection sensitivity dC/dt of the potential sensor is proportional to the effective area of the detection electrode. FIG. 11 is a cross-sectional view along a line 11-11 in FIG. 10. Let it be assumed that a detection electrode 1021 has a width W1, an interval between the detection electrodes is a length W2, an aperture 1024 has a width W3, and a mask portion 1026 has a width W4. In order that the detection electrode can be exposed completely, width W3 of the aperture has to be equal to or larger than width W1 of the electrode (W3≧W1). In order that the detection electrode can be masked completely, first, width W4 of the mask portion has to be equal to or larger than width W1 of the electrode (W4≧W1). In addition, the interval W2 has to be equal to or larger than width W3 of the aperture. On the other hand, a condition of the widths for efficiently exposing and masking the detection electrode with a minimum moving distance of the movable shutter is given by setting up the widths and interval equal to one another. Consequently, widths W1 and W2 are almost equal to each other so that the effective area of the detection electrodes has been limited to about a half of an occupied area on the substrate.
The present invention has been made in consideration of the aforementioned situation.